In taking a loop at the comparable loop systems, horizontal is probably going to make the most sense for us in KS since we have plenty of room.  The real issue is how deep we need to go to get into nice, level temp soil.  I know our water right now is at 58* and I am about to go drill some test holes just for fun...  Most locals are installing at a depth of 6ft but I would much prefer to go deeper. 

As well, thermal conductivity of soil sucks.  Moist to saturated soils nearly double in conductivity.  I was originally thinking about putting metallic pipe in the ground but with the conductivity values, it just makes little sense.  PE pipe is not very thermally conductive but slightly better than soil and on par with water.  One thing that I think can really help when installing a horizontal loop is the addition of either a hydro gel or other water absorbing media or the use of some other higher conducting media.  Due to the low thermal conduction of both the pipe and soil, adding a media between them to increase mass and surface area to take better advantage of the large amount of mass of the earth. 

I am curious if anyone has any information or experience on the subject or maybe any soil data for my area? 

Another large obstacle is hitting my target trench depth economically.  Being in the heavy construction business as well, some equipment is available to me but trenchers that will hit 8-10ft and make a nice floor are SERIOUS machines.  In those cases, we would usually reach for a track hoe but with so much trench to do, the thought really comes in to maybe even rent something for this event.  I certainly will not be hiring anyone to do the job.  just laid way too much pipe to call this a difficult job. 

Good catch lunderstanding that metal does no extra good in most soils. HPDE is cheap, tough, durable, and conducts enough to not be the weak link.

Another issue is trench shoring - I believe shoring rules kick in at 6'+, greatly increasing costs. It may be that for same cost you can have 2-3x the trench length at 6' vs 8-10 for relatively little difference in performance.

I believe you have to get to 30+' to reach soil with no seasonal temperature variation. This is the appeal of vertical loops, but drilling generally costs much more than trenching.

Consolidation of new horizontal systems is sometimes a problem - some folks have watered the heck out of them or provided soaker hoses part way down for this. I've read some systems take a year or more.

Adding some sort of more highly conductive media is an interesting idea, but I doubt it is feasible owing to the massive volume likely needed - too expensive.

Shoring is actually required over 4ft but shoring is only required if someone will be in the hole. I would not have to deal with inspections on this so if we shot to 8-10ft, I would just throw it in the hole, move it around with a stick, and go.

Regarding conductive medias. I was thinking of using something cheap that conducts well. Reading up on the grout that is used for vertical wells, Bentonite is very common in wells anyway for plugging. The stuff is cheap and seems to have reasonable conductivity BUT I might look around for something even better. Obviously a lot will be needed so tandem load pricing needs to be reasonable.

Guess I really need to learn temperatures in the ground before I can move forward... If I cannot find what I need, I might just fab up some thermocouples and do some test holes, etc. I drilled to 4ft at home and got 76*. This is with temps hitting record highs every single day for the last 2 months. It could not be a better season to test the hot season. However, 76* just will NOT cut it. I would not waste my time for that temp. I wanna see 60* with a 2.5* delta...

Bentonite has a conductivity of .40, almost any soil will be better.

if you are 76° at four feet, you will not see 60° at 10'

I am not too familiar with Bentonite but the grout I was referring to had a conductivity of 4-6. Not super impressive but I guess that is why I am here.

Regarding the soil temps, I am not a geologist but I guess I am banking on our rather shallow water table which has proven water temps in the 50s at 20ft to help a bit. I also was looking at some data from another design firm in our area that speculates that at 6ft, our temps are pretty stable at 60. I would have to admit that my method of testing was not super precise.

As an "ass guide", I used to help install sewer lines 20 yrs ago and remember climbing in the 8-9ft trench and feeling pretty refreshed in a hot summer day. Obviously I did not have the notion to test the temperature but cooler than room temp for sure.

Anything you do to increase thermal conductivity helps - metal pipe, good grout, soaker hose, larger pipe, etc. Thermal resistances add without "limiting factors" - just decreasing importance. The question is what is cost effective to do.

The soil temperatures that you measure before installation are a lower/upper bound, but remember, it will rise/fall considerably as you add/remove heat. So don't get too hung up on measuring undisturbed temperatures. A little more pipe at 6' may outperform 10' at far lower cost.

If you are set on going deep, then open loop, vertical or horizontal boring may be better choices than trenches.

Posted By fastline on 30 Jul 2011 12:44 AM
I am not too familiar with Bentonite but the grout I was referring to had a conductivity of 4-6. Not super impressive but I guess that is why I am here.

Not sure what you are referring to.  Even it you buy the best bentontite grout at $14 for 50 lbs  and add  200 lbs.  of high grade silica sand  at $75 to  enhance the TC,  you end up with a TC of 1.2.   I would like to know more about the grout that can deliver a TC of 4-6

The measured temps in undisturbed soil at various depths are interesting but not particularly relevant to a working system sourcing / sinking 100s kBTU / day at whatever depth you ultimately choose.

If deep ground temp is 58, a good rule of thumb is to shoot for a 20 deg delta under design conditions. In other words, look for 38 deg EWT on a design winter day and 78 in summer. Both temps will provide very high efficiency.

A partner/designer in your project could actually show you the different benefits of assorted depths and trench lengths and you can still do all the work yourself.
Your first step is sizing the equipment, not designing the loop field.
j

Posted By joe.ami on 31 Jul 2011 09:54 AM
A partner/designer in your project could actually show you the different benefits of assorted depths and trench lengths and you can still do all the work yourself.
Your first step is sizing the equipment, not designing the loop field.
j

Wow, I will admit that I have not put any calcs together for this but I really figured designers were looking to get much closer to soil temp.  Obviously any effort to get to that median temp would be rewarded with higher eff. 

Maybe you can clarify but as I am looking at this, the soil can work well for heap pump applications but because of the poor conductivity, the heat does not diffuse readily so EWT during cooling months will actually go up and you try to sink more and more BTUs in the soil?  With that data, I can sure see why a water source with water to water exchanger might be far superior in terms of efficiency. 


Realizing that the PE pipe can be a bottleneck in the exchange here, I am still surprised no one is using a exchange fluid conductivity booster.  Maybe that has been tried and did not work out?  Probably ultimately at the mercy of the dirt I guess. 

You should download a free copy of "Ground Loop Design" and try different variables and note the effect.

Thanks. I did take a look at that software but looks like they are directing me to leave contact info so they can give me a "tour". While I would certainly be excited to plug through some software for data, I really also need to put my own hard calcs together so I can better understand "why". It would certainly be interesting to see how the variables change the performance of the system.

I do have a good friend that is an HVAC design engineer so hopefully he can shed some light on some of this through experiences.

Posted By fastline on 31 Jul 2011 11:24 PM

I do have a good friend that is an HVAC design engineer so hopefully he can shed some light on some of this through experiences.

If your friend is not practiced in geo, he/she is not a good choice for advice as rookies tend to select a system that covers 100% of the load while the industry norm is to cover 92-98% of the load and use auxiliary for the balance.


"Wow, I will admit that I have not put any calcs together for this but I really figured designers were looking to get much closer to soil temp.  Obviously any effort to get to that median temp would be rewarded with higher eff."

You simply can not design a loop field without knowing how many btus you need to collect and discharge.

good luck,
joe